Halftone reproduction is a well-known method of producing an image by displaying variable sized dots on a reproduction surface. By varying the size of the dots, a human eye can be made to perceive a desired image in the reproduction. In most halftone reproductions, the locations of the dots, called pels (short for "picture elements " ) are fixed or raster points.
An understanding of halftone reproduction is assisted by a discussion of halftone printing. In a typical black-and-white halftone printing system, an image to be reproduced is photographed through a screen which divides the image into a large number of halftone cells. The size and center-to-center spacings of these halftone cells are controlled by the screen dimensions. The resulting photographic negative is used to create a printing plate comprised of dots located at the center of the halftone cells and having sizes dependent upon the amount of black in the original image at the halftone cell locations. The printing plate dots are then inked and used to imprint a reproduction surface, thereby reproducing the image. If the space between dot centers is sufficiently small, the reproduction appears continuous to the naked eye. The quality of the reproduction depends upon the proper matching of such variables as the reproduction medium, the inks used, the dot center-to-center spacings, and the method of applying the ink onto the reproduction surface.
Color halftone printing systems are similar to black-and-white halftone printing systems. Typically, inks of three colors, usually cyan, yellow and magenta (CYM), are used. However, because of impurities in commercially available CYM inks, black ink is frequently added to assist in reproducing shades of black, producing what is called a CYMK system. Color halftone printing systems involve photographing an image through separate color screens set at different screen angles for each color and using a different color filter for each system color. The resulting color negatives are used to produce separate color printing plates. These plates are then inked with their appropriate colored ink and used to imprint the reproduction surface. As with black-and-white halftone printing, the quality of the reproduction depends upon the proper matching of such variables as the reproduction medium, the inks, the color dot center-to-center spacings, and the various amounts of the colored inks applied. However, many other factors such as the screen angles used and the match between the color filters and the ink colors are important in color halftone reproduction systems. Because of the sensitivity of the human eye to color hues, proper matching between all factors is more critical.
Color halftone reproduction has been adapted for use with digital computers, CRTs, and color printers. Prior art digital color halftone reproduction systems have used image scanners to digitize an original image; have included networks for altering the color hues of the image; have incorporated CRTs for enabling an operator to observe the effects of the color alterations; have included methods for simulating various screen angles; and have included methods for calculating the amounts of each color ink required to correctly reproduce the image. However, as personal computer systems and low-cost color printers have become more common, a need has developed for a system of creating high-quality halftone reproductions using these components.
In prior art color halftone printing systems the individual color screens are orientated at different screen angles to avoid excessive overlap of the various inks. FIG. 1 illustrates the prior art technique of using multiple screen angles. While different screen angles may be used, the yellow dots are frequently printed at a yellow screen angle 2 of 90.degree., the black dots at a black screen angle 4 of 45.degree., the magenta dots at a magenta screen angle 6 of 75.degree., and the cyan dots at a cyan screen angle 8 of 105 .degree.. These particular screen angles have been found to assist in creating a pleasing reproduction image. However, the resulting ink dots of different colors have variable center-to-center spacings and overlap seemingly at random. This variability produces some areas in the reproduction having high dot densities and a lot of ink and other areas with low dot densities and little ink. If these dot variations become visible, a highly objectionable interference effect known as a moire pattern is observed.
Prior art halftone color printing methods produce high-quality color prints without visible moire patterns if the center-to-center spacings between color dots of the same color is small, say less than about 1/1200th dots per inch. As the spacings and dots become larger, the moire, pattern becomes more pronounced and, consequently, the quality of the reproduction deteriorates. Low-cost color printers such as those often used with personal computers, typically can print no more than about 300 dots per inch, resulting in highly visible moire patterns using prior art techniques.
Another problem with prior art color halftone printing systems is that the calculations of how much of each color ink to deposit on the reproduction surface to accurately reproduce an image, the so-called color separation calculations, are very involved. In part, this is because the various color dots use different screen angles and thus interact in a hard-to-determine manner. Since the central processing units (CPUs) of common personal computers are relatively slow, and users want printing to occur with little or no delay, the required calculations are not performed as quickly as is often desirable.
Therefore, there has existed a need for a method and system of performing color halftone reproduction such that personal computers and low-resolution printers and CRT displays can produce acceptable images and such that the computational complexity of the ink density calculations, or color dot density calculations when the reproduction surface is a CRT, is reduced.